1 hernial Hysteresis of Fused Silica. 727 



is the linear expansion for a rise of temperature of t°, H may- 

 be looked upon as the residual variation of the expansion 

 coefficient. 



Specimens of annealed fused silica both clear and trans- 

 lucent were obtained in the form of end-measure rods about 

 45 cms. long. They were subjected for periods of from 1 to 

 90 hours to various temperatures over a range of from about 

 — 190° to 400° C. For annealed specimens, H, as defined 

 above, came out between 1 X 10~ 9 and 5 x 10~ 9 , which is less 

 than 1 per cent, of the expansion coefficient at ordinary 

 temperatures. Usually H was negative, which means that 

 after heating and then cooling to the original temperature, 

 the contraction was greater than the preceding expansion. 

 This is in accordance with the observations of Minchin and 

 of Randall, and is, of course, one of the characteristics of 

 invar. 



As will be seen below, silica compares very favourably 

 with the two common Jena thermometry glasses specially 

 designed to show a small after-effect. To extend the com- 

 parison, Guillaume's figures for invar are added. 



Substance. 



Jena 16' " * 

 Jena 59' " t 

 Invar \ 



Fused Silica 



Temp. Kange. i Linear Hysteresi s H. j Authority. 



Thiesen & Scheel. 



0° to 50° 



to 50 



to 25 



to 50 



to 400 



to 800 



to >1000 



42 Xl0 

 23 



-81 

 - 162 



-1 to — 5 ,, 

 -17 



-f- values 



Guillaume. 

 N. P. L. 

 Callenclar. 



* H=(34+0-16*) 10 -9 . t H=(27-0 08010" 9 . J H=-3-25*xl0- 



Thus in regard to linear after-effect, silica over the range 

 •0° to 400° has nothing to fear in comparison with either 

 invar or Jena thermometry glasses subjected, as will be seen, 

 to much less severe temperature conditions. There is prac- 

 tically nothing to choose between the different kinds of fused 

 silica ; the cheaper satin-like variety is as good as the more 

 expensive clear transparent kind. 



In justice to invar, it ought to be added that G-uillaume's 

 observations on after-effect extended over months, while the 

 measurements on silica were included in the space of a few 

 hours. 



